1. Field
The present disclosure relates generally to data networking and in particular to a backhaul radio for connecting remote edge access networks to core networks with advantageous spectrum usage.
2. Related Art
Data networking traffic has grown at approximately 100% per year for over 20 years and continues to grow at this pace. Only transport over optical fiber has shown the ability to keep pace with this ever-increasing data networking demand for core data networks. While deployment of optical fiber to an edge of the core data network would be advantageous from a network performance perspective, it is often impractical to connect all high bandwidth data networking points with optical fiber at all times. Instead, connections to remote edge access networks from core networks are often achieved with wireless radio, wireless infrared, and/or copper wireline technologies.
Radio, especially in the form of cellular or wireless local area network (WLAN) technologies, is particularly advantageous for supporting mobility of data networking devices. However, cellular base stations or WLAN access points inevitably become very high data bandwidth demand points that require continuous connectivity to an optical fiber core network.
When data aggregation points, such as cellular base station sites, WLAN access points, or other local area network (LAN) gateways, cannot be directly connected to a core optical fiber network, then an alternative connection, using, for example, wireless radio or copper wireline technologies, must be used. Such connections are commonly referred to as “backhaul.”
Many cellular base stations deployed to date have used copper wireline backhaul technologies such as T1, E1, DSL, etc. when optical fiber is not available at a given site. However, the recent generations of HSPA+ and LTE cellular base stations have backhaul requirements of 100 Mb/s or more, especially when multiple sectors and/or multiple mobile network operators per cell site are considered. WLAN access points commonly have similar data backhaul requirements. These backhaul requirements cannot be practically satisfied at ranges of 300 m or more by existing copper wireline technologies. Even if LAN technologies such as Ethernet over multiple dedicated twisted pair wiring or hybrid fiber/coax technologies such as cable modems are considered, it is impractical to backhaul at such data rates at these ranges (or at least without adding intermediate repeater equipment). Moreover, to the extent that such special wiring (i.e., CAT 5/6 or coax) is not presently available at a remote edge access network location; a new high capacity optical fiber is advantageously installed instead of a new copper connection.
Rather than incur the large initial expense and time delay associated with bringing optical fiber to every new location, it has been common to backhaul cell sites, WLAN hotspots, or LAN gateways from offices, campuses, etc. using microwave radios. An exemplary backhaul connection using the microwave radios 132 is shown in FIG. 1. Traditionally, such microwave radios 132 for backhaul have been mounted on high towers 112 (or high rooftops of multi-story buildings) as shown in FIG. 1, such that each microwave radio 132 has an unobstructed line of sight (LOS) 136 to the other. These microwave radios 132 can have data rates of 100 Mb/s or higher at unobstructed LOS ranges of 300 m or longer with latencies of 5 ms or less (to minimize overall network latency).
Traditional microwave backhaul radios 132 operate in a Point to Point (PTP) configuration using a single “high gain” (typically >30 dBi or even >40 dBi) antenna at each end of the link 136, such as, for example, antennas constructed using a parabolic dish. Such high gain antennas mitigate the effects of unwanted multipath self-interference or unwanted co-channel interference from other radio systems such that high data rates, long range and low latency can be achieved. These high gain antennas however have narrow radiation patterns.
Furthermore, high gain antennas in traditional microwave backhaul radios 132 require very precise, and usually manual, physical alignment of their narrow radiation patterns in order to achieve such high performance results. Such alignment is almost impossible to maintain over extended periods of time unless the two radios have a clear unobstructed line of sight (LOS) between them over the entire range of separation. Furthermore, such precise alignment makes it impractical for any one such microwave backhaul radio to communicate effectively with multiple other radios simultaneously (i.e., a “point to multipoint” (PMP) configuration).
In particular, “street level” deployment of cellular base stations, WLAN access points or LAN gateways (e.g., deployment at street lamps, traffic lights, sides or rooftops of single or low-multiple story buildings) suffers from problems because there are significant obstructions for LOS in urban environments (e.g., tall buildings, or any environments where tall trees or uneven topography are present).
FIG. 1 illustrates edge access using conventional unobstructed LOS PTP microwave radios 132. The scenario depicted in FIG. 1 is common for many 2nd Generation (2G) and 3rd Generation (3G) cellular network deployments using “macrocells”. In FIG. 1, a Cellular Base Transceiver Station (BTS) 104 is shown housed within a small building 108 adjacent to a large tower 112. The cellular antennas 116 that communicate with various cellular subscriber devices 120 are mounted on the towers 112. The PTP microwave radios 132 are mounted on the towers 112 and are connected to the BTSs 104 via an nT1 interface. As shown in FIG. 1 by line 136, the radios 132 require unobstructed LOS.
The BTS on the right 104a has either an nT1 copper interface or an optical fiber interface 124 to connect the BTS 104a to the Base Station Controller (BSC) 128. The BSC 128 either is part of or communicates with the core network of the cellular network operator. The BTS on the left 104b is identical to the BTS on the right 104a in FIG. 1 except that the BTS on the left 104b has no local wireline nT1 (or optical fiber equivalent) so the nT1 interface is instead connected to a conventional PTP microwave radio 132 with unobstructed LOS to the tower on the right 112a. The nT1 interfaces for both BTSs 104a, 104b can then be backhauled to the BSC 128 as shown in FIG. 1.
The conventional PTP radio on a whole is completely unsuitable for obstructed LOS or PMP operation. To overcome, these and other deficiencies of the prior art, one or more of the inventors has disclosed multiple exemplary embodiments of an “Intelligent Backhaul Radio” (or “IBR”) in U.S. patent application Ser. No. 13/212,036, now U.S. Pat. No. 8,238,318, and Ser. No. 13/536,927, which share a common assignee to this invention and are hereby incorporated by reference in their entirety into this invention.
Both conventional PTP radios and IBRs can be operated using time division duplexing (or “TDD”). FIG. 2 depicts a generic digital radio, not necessarily a conventional PTP radio or an IBR, in a TDD configuration. With TDD, only one of the radios amongst the two peers depicted in FIG. 2 transmits at any given point in time according to a time schedule known to both radios. This enables both TDD radios to transmit and receive at the same frequency or within the same finite frequency band. Thus, relative to FIG. 2, each radio transmits and receives at a single nominal carrier frequency f1, but when one radio in any given link (of which only one link is depicted in FIG. 2) is transmitting, then the other radio is receiving wherein neither radio transmits and receives at the same point in time.
Both conventional PTP radios and IBRs can be operated using frequency division duplexing (or “FDD”). FIG. 3 depicts a generic digital radio, not necessarily a conventional PTP radio or an IBR, in a conventional FDD configuration. With conventional FDD, each of the radios amongst the two peers depicted in FIG. 3 transmits at separate frequencies, f1 and f2, known to both radios but chosen such that frequency-selective filters within the transmitters and the receivers of each radio can sufficiently reject unwanted transmit noise and signal leakage within each receiver. In FDD, both radios amongst the two peers depicted in FIG. 3 can transmit and receive simultaneously.
There are well-known advantages and disadvantages for each of TDD or FDD operation for both the generic radios of FIGS. 2 and 3, or for IBRs or conventional PTP radios. Many advantages of TDD are related to disadvantages of FDD, or vice versa.
For example, in TDD antenna resources are easily shared between transmit and receive, which is a distinct advantage for TDD. Conversely, in FDD antenna resources are not easily shared and either separate antennas for transmit and receive or a frequency duplexer is required to support conventional FDD radio operation. For example, in utilizing TDD, the isolation of the receiver from the intentional or unintentional signals of the transmitter is straightforward since transmission occurs only when reception does not occur, and vice versa. Conversely, in utilizing FDD, the transmitter signals (both intentional and unintentional) must be highly isolated from the receiver channel, which is a disadvantage for FDD such that conventional FDD radios have required multiple additional filters relative to TDD as well as separate antennas or frequency duplexers. For example in TDD, spectrum access is very flexible as any portion of the radio spectrum that supports the desired channel bandwidth can theoretically be used and the duty cycle between transmit and receive can be varied easily, which is another distinct advantage for TDD. Conversely, in FDD spectrum access channels for transmit and receive need to be paired at frequency separations compatible with the isolation requirements for conventional FDD radio operation.
However, even conventional FDD radios have significant advantages over TDD despite the disadvantages described above. For example in FDD, with separate antennas between transmit and receive, the losses between either the power amplifier and the transmit antenna or the receive antenna and the low noise amplifier are lower than that of a TDD radio with a transmit/receive switch, which is an advantage for such an FDD radio. As another example in FDD, very high PHY and MAC efficiency can be obtained in a backhaul radio with simultaneously achieving very low latency in transmission and reception without any significant buffering at the network interface, which is a distinct advantage for FDD. Conversely, in TDD a trade-off is required between PHY and MAC efficiency on one hand and minimizing latency and buffering on the other hand. For example, in FDD all of the “non-antenna” resources, such as the entire transmit path and the entire receive path are utilized at all times. Conversely, in TDD much or all of the transmit path may be idle when receiving and vice versa for the receive path when transmitting, which significantly underutilizes the resources within a TDD radio. For example, in FDD a low latency feedback channel to each transmitter is available because a peer FDD radio does not need to wait until such transmitter stops transmitting to send feedback information that may enhance the link performance. Conversely, in TDD no feedback can occur until the transmitter stops and the radio goes to receive mode and thus feedback latency must also be traded off against overall efficiency.
Recently, it has been proposed that certain types of radio systems that operate within a single channel or band may transmit and receive simultaneously without all of the isolation circuitry and frequency separation of conventional FDD systems by adding a cancellation capability as shown conceptually in FIG. 4. In such an exemplary configuration, which is referred to herein as “Zero Division Duplexing” (or “ZDD”), each radio in a particular link transmits and receives at single nominal carrier frequency f1 as is allowable for TDD, but can transmit and receive simultaneously as is allowable for FDD Such a ZDD radio has many of the advantages of FDD, such as for example only, low latency and high efficiency, as well as many of the advantages of TDD, such as for example only, flexible spectrum access. Furthermore, a ZDD radio in theory can have at least twice the single-link spectral efficiency of either an FDD or TDD radio by utilizing the same frequency channel simultaneously in both directions of a link. In some configurations, ZDD operation may be at two channels about a nominal frequency f1 such that no conventional isolation filtering at RF is possible in which case the above referenced advantages still apply except for the spectral efficiency improvement. As shown in FIG. 4, exemplary ZDD radios have attempted to cancel unwanted transmitter leakage primarily by inverting the phase of an attenuated copy of the transmit signal and summing it at the receiver to substantially cancel the transmit signal leakage at the receiver antenna.
However, known techniques for implementing ZDD radios are completely inadequate for an IBR, or many other high performance antenna array based radio systems, to operate in any ZDD mode whether “co-channel” or “co-band”.